Many 18th century paintings are known for their realism, but the work of the Italian painter Canaletto is so precise that scientists are using it to document sea-level change in Venice. The paintings date back 140 years before the city began keeping official records of sea level.

To meet the voracious demand for high-end souvenirs of Venice, Canaletto used a portable camera obscura to crank out a picture-perfect painting of Venice about every 3 days. He kept up this frantic pace for more than a decade during the 1730s and 1740s, says Dario Camuffo, a climatologist at the Italian National Research Council in Padua.

The artist was so accurate that he even captured a greenish-brown layer of Laminaria algae lining the canals. Because Laminaria float on the surface of the water, this layer indicates average high tide, allowing Camuffo to calculate the rate of increase of the sea level in the Venice Lagoon as 2.7 mm per year. This fits with official measurements begun in 1871, which show a rise of 2.4 mm per year. "I was surprised that these two kinds of data fit so well," says Camuffo, who examined more than 100 of Canaletto's paintings. "It was astonishing." Knowing the earlier rate could help predict future sea-level change. Camuffo presented his findings in Venice on 9 October at a research briefing on protecting Europe's cultural heritage.

Camuffo's results appear reliable, although they only apply locally, says Frank Oldfield, former director for Past Global Changes at the International Geosphere-Biosphere Program in Stockholm. To see if the oil-painting data are part of a global trend, he adds, they would need to be examined alongside similar records of sea-level change from other parts of the world. Such data, though, have yet to be gathered.